Conventionally, an ordinary microscope (meaning one that does not provide stereo viewing optics so that a 3-D image is viewable) as shown in FIG. 8 may be formed of a lamp housing 1 having a light source 2 that generates illumination light; a collector lens 3 that converts light from the light source 2 into a substantially collimated light; a light-balanced daylight filter 9 that causes the spectral content of the light emitted by the light source to more closely resemble that of daylight; a diaphragm 4 that functions as a field stop to restrict the illumination light; a field lens 5 that converts the substantially collimated light emitted from the lamp housing 1 to a collected light flux; an aperture diaphragm 6; and a condenser lens 7 through which the illumination light from the field lens passes to a sample surface 8 that may be composed of an illuminated surface of an object.
In recent years, as a light source used for an illumination optical system, attention has been given to using solid-state lighting elements, such as LEDs and the like. An LED has the advantage of consuming less electrical power and having a longer life span than conventional halogen light sources. In particular, as a light source substitute for conventional halogen light sources, LEDs that emit light that is perceived as being white in color (hereinafter termed white light LEDs) have been developed. For example, Japanese Patent 2002-543453 and Japanese Laid Open Patent Application 2005-148296 each disclose an illumination optical system that uses a white light LED.
In biology/medical treatment applications where pathological specimens (i.e., thin slices) are examined using a microscope provided with an illumination optical system, a determination is made concerning disease based on the color of the observed cell specimen. During such examinations, if the spectral distribution of the illumination light source is changed, even when observing the same specimen using a microscope, the color of the observation object that may be observed either visually, or through a solid-state imaging element such as a CCD or the like, may result in erroneous diagnostic results. Owing to this, in an illumination light source used for diagnosis, it is important that there be a rather uniform distribution of spectral components in the illumination light. Thus, even when using a novel light source such as a white light LED, it is desirable to make the spectral components of the illumination light more closely resemble the spectral components in daylight by combining a white light LED light source with a wavelength distribution conversion element so as to thereby provide a distribution of spectral components that is similar to that output by a conventional halogen light source when combined with a light balanced daylight filter (for example, the spectral output shown by the dash line in FIG. 10).
A single lighting element of an LED typically provides light having a narrow spectral distribution centered about a single peak wavelength. Also, a white light LED may be composed of a single LED lighting element that provides a narrow spectral distribution centered about one peak wavelength and a fluorescent element that emits light having wavelengths centered about a longer peak wavelength. The fluorescent element is excited by the light emitted by the LED. White light is observed due to the eye being unable to resolve two separate point light sources located close together, and so the two light outputs are effectively mixed together and appear as a single white light source.
However, in the case of a solid-state lighting element that uses a white light LED, one can see by comparing the curve of FIG. 9 with the solid curve of FIG. 10 that the spectral distribution emitted by such a white light LED (having one peak wavelength near 450 nm and another peak wavelength near 550 nm, as shown in FIG. 9) differs substantially from the spectral distribution of a halogen light source (shown by the solid line in FIG. 10) that is conventionally used as the illumination light source.
Owing to this, an illumination optical system that is equipped with a solid-state lighting element to generate white light (such as disclosed in JP 2002-543453 and in Japanese Laid Open Patent Application 2005-148296) cannot be used in its existent state for the diagnosis of pathological segmentations, and the like, using a microscope that is equipped with a halogen light source, since the diagnostic standards required when using illumination of an observation body with a microscope that employs a halogen light source are different than when using illumination of an observation body with a microscope that employs a solid-state lighting element. For example, if the illumination light source of a microscope were to be switched to a white light LED from a conventional light source, this would require that the microscopic diagnosis must be performed using a different diagnostic standard than previously used. Thus, an increased burden would be placed on individuals who perform diagnoses, owing to the concern about the possibility of an error in the diagnosis occurring due to an inapplicable diagnostic standard being applied for a given light source type.
In industrial applications as well, when mixing an examination performed using a microscope having a conventional halogen light source with an examination performed using a microscope having a white light LED (for example, in the case of using color to detect defect(s) of a magnetic head formed from multiple layers), there are inconveniences that are similar to those found in biological and medical applications.